In known injection systems, the nozzle needle of the respective injection valve is actuated by means of a piezo actuator. When an electrical voltage is applied to the piezo actuator, the latter increases its length with the result that in this way the position of the nozzle needle of the injection valve can be controlled. For such injection valves, very stringent requirements are made of the accuracy and robustness of the injection quantity under all operating conditions and over the entire service life of the respective vehicle.
In order to identify the nozzle needle position during the actual injection process (active operation), feedback signals from the piezo actuator are used. The piezo actuator acts here as a sensor. This information is, however, subject to large interference variable influences because the piezo actuator is used simultaneously as an actuator and as a sensor. Interference variables relating to the feedback signal of the piezo actuator are generated, inter alia, by the actuation profile of the associated output stage for the application of current, by the idle stroke in the transmission of force between the piezo actuator and the nozzle needle, by friction effects in the region of the nozzle needle and by the actual stroke behavior of the piezo actuator. All these influences can act as interference variables on the feedback signals and therefore cause the connection of control variables for the correction of injection quantities to be falsified. These influences therefore reduce the robustness of the derived control variables and therefore also affect the quality of the control.
Previously, this problem has been solved by feedback evaluation algorithms for detecting the nozzle needle position using various threshold values from the electrical signals of the piezo voltage or piezo charge in order to carry out plausibility checking of the closing and/or opening information. However, in this context the robustness depends on the selected threshold values which, however, under certain circumstances cannot filter out all the possible embodiment failures or fault failures and therefore give rise to enduring, unacceptable residual faults.